1. Field of the Invention
This invention relates to a cylinder head for use in an internal combustion engine and the like, and more particularly to a port liner which covers over the wall of a cavity in the cylinder head for insulating heat to be transmitted from exhaust gases to the wall of the cylinder head.
2. Description of the Prior Art
It has been common practice to use a port liner for a cylinder head in an attempt to insulate the heat being transmitted from exhaust gases to the cylinder head, which is being cooled with cooling water. This is because the temperature of exhaust gases should be maintained at a high temperature for effecting recombustion of exhaust gases in an exhaust system of an internal combustion engine, thereby purifying exhaust gases containing unburnt gaseous components.
Accordingly, such a port liner should be made of a metal having high heat resistance and relatively low heat conductivity. The port liner is installed within a cavity in a cylinder head in a manner to define an annular space between the wall of the cavity in the cylinder head and the port liner. Thus, the heat of exhaust gases flowing through the port liner may be insulated by means of the annular space defined therearound, so that the heat from exhaust gases will not be unfavorably transmitted to the cylinder head.
Meanwhile, the cavity or exhaust gas passage in the cylinder head, in general, follows a complex route, so that a port liner is generally formed into two halves, upper and lower, in the course of fabrication, and then those upper and lower halves are welded together along suitable edges or flanges to give a port liner of an integral construction.
Hitherto, the port liner is installed in the cylinder head by using the so-called "insert" or "filler" casting technique. More particularly, the port liner thus welded and hence of an integral construction is buried in a sand core beforehand, and then the core is placed in a cavity defined within the upper and lower halves of a casting mold, i.e., a cope and a drag, after which a melt is poured therein. Thus, the end of the port liner on one side is embedded in the inner wall of the cylinder head thus cast, while the other end of the port liner is freed. After cooling, the core sand is removed, thereby providing the annular space between the wall of a cavity in the cylinder head and the port liner. Then, the other end of the port liner is welded to the wall of a cavity in the cylinder head or sealed with a sealing part to confine the annular space completely. Such an attempt therefore dictates the use of a sealing part or welding, suffering from shortcomings of insufficient sealing accruing from the dimensional error or allowances of the sealing part and the port liner, and excessive man hours required for the fabrication of the cylinder head.